Comparison of Polynuclear Aromatic Hydrocarbon ... - ACS Publications

10

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 25, 2017 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch010

Comparison of Polynuclear Aromatic Hydrocarbon Cation Salts with Salts of Simple Fluoroaromatic Cations Thomas J. Richardson, Francis L . Tanzella, and Neil Bartlett Department of Chemistry, University of California at Berkeley, and the Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720

Electron oxidation of thefluoroaromaticcompounds C F and C F by O2+ salts yields salts of the radical cations C F6+ and C F which are Curie law paramagnets. Thefluoroanaloguesof the metallic (C H )2 salts do not exist. Repulsive interactions involving the electron-richfluorineligands of thefluoroaromaticcompounds are probably responsible for the failure of these species to make metallic stacks. Attempts to prepare C H6+ salts have produced the poly(p -phenylene) cation salts (C H )n AsF -, which are good electronic conductors. Electron oxidation of polynuclear aromatic hydrocarbons by C F6+ salts or by AsF , (e.g., 3AsF + 2C H12 —> 2 C 2 4 H 1 2 + A s F 6- + AsF ) yields what appear to be salts of the polynuclear aromatic cations. The magnetic and electrical properties of such salts are described. 6

6

10

6

10

8

8

10

8+

+

6

6

6

4

+

6

5

24

5

3

RADICAL-CATION

SALTS derived from hexafluorobenzene (1), octafluoro-

toluene (2), pentafluoropyridine (3), and octafluoronaphthalene (4) have been known for some time, and some of their reaction chemistry has been dis cussed i n a recent publication (5). Hexafluorobenzenehexafluoroarsenate, C F A s F - , has oxidizing power sufficient to electron oxidize most other mono- and polycyclic aromatic compounds. Fritz et al. (6) prepared bis(naphthalene) salts, ( C H ) MF - (M = 6

6

+

6

1 0

8

2

+

0065-2393/88/0217-0169$06.00/0 © 1988 American Chemical Society

Ebert; Polynuclear Aromatic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

6

170

POLYNUCLEAR AROMATIC COMPOUNDS

Ρ, As) in which the aromatic molecules occur in stacks; this situation results in metal-like electrical conductivity (σ) (σ = 0.12 ± 0.046 Ω " c m for a polycrystalline pellet). This finding suggested the possibility of analogous behavior in the fluoroaromatic series. Materials containing dimeric cations, however, have not been isolated from reaction mixtures containing excess amounts of the neutral monomers or from controlled reduction of monocation salts. In each case, the cations are monomeric and magnetically independent of one another. Attempts to prepare salts containing C H have led to polymerization with H F elimination, and the resulting solid contains electron oxidized poly(p-phenylene) (7). Thermally stable blue-green powders have been obtained by the oxi dation of coronene with 0 A s F , C F A s F , or A s F . IR spectra of these materials show the presence of the A s F " ion in addition to the coronenelike cation. Gravimetry and elemental analyses indicate compositions ranging from ( C H ) A s F to ( C H ) A s F ; at least three crystallographically distinct phases are indicated. In the X-ray powder diffraction patterns of these solids very strong reflections with d spacings of about 3.3 A suggest that the coronene species may be interplanar stacked in a platelike fashion. Crude resistance measurements on pellets of the polycrystalline powders indicate ambient-temperature conductivity for these salts in excess of 1.0 X lOÔêm" .


1

6

2

6

6

6

6

- 1

+

6

5

6

2 4

1 2

4 0

6

2 4

1 2

0 2 5

6

1

Experimental Details The syntheses of cation salts of the monocyclic fluoroaromatic compounds and of oetafluoronaphthalene have been described elsewhere (1-5). Manipulations of airor moisture-sensitive materials were carried out in a Vacuum Atmospheres Dri-Lab or in a stainless steel vacuum line fitted with a Teflon FEP[poly(tetrafluoroethylene)] reaction vessel (Chemplast) or a fused silica reaction vessel. Reaction of Benzene with 0 A s F . In a typical reaction, benzene (0.403 g, 5.16 mmol) was co-condensed at 77 Κ with sulfuryl chloride fluoride (8 mL) into a Teflon F E P reaction vessel containing 0 A s F (0.912 g, 4.13 mmol, prepared as in reference 8). On warming to 195 K, a green solution was obtained from which oxygen evolved steadily for a period of 15 min as the color faded. When the solvent and volatile products were removed at room temperature, a dark-brown solid (0.893 g) remained. The product was washed with liquid anhydrous hydrogen fluoride to remove (C H5)2AsF AsF6, which was formed by the reaction of benzene with AsF present in the reaction mixture as a result of the thermal decomposition of 0 A s F . The resulting brown powder (0.182 g) was diamagnetic and had a room-temperature conductivity (pressed pellet) in excess of 1.0 Χ 10~ Ω c m . The analysis was the following: [(C H )„AsF6, η = 4.05, based on the C/As ratio] C , H , As; F : calculated, 21.05; found, 22.07. The IR spectrum (Figure 1) of the powder contains absorptions due to oxidized poly(p-phenylene) (9) and the hexafluoroarsenate(V) ion. 2

6

2

6

6

2

5

2

2

6

1

6

- 1

4

Reaction of Benzene with CeFeAsFe. CeFeAsFe was prepared in situ by re acting 0 A s F (0.473 g, 2.14 mmol) with an excess of C F in S0 C1F prior to the 2

6

6

6

2


10.

RICHARDSON ET AL.


1600

1400

171

Comparison of PAH Cation Salts

1200

1000

800

600

400

Figure 1. IR spectrum of oxidized poly(p-phenylene).

addition of benzene (0.323 g, 4.14 mmol). The reaction was complete in 1 h at 195 K. The product (0.546 g, 0.134 g after washing with HF) was identical to that produced from 0 A s F . Elemental analyses of samples from four preparations gave values for η ranging from 1.8 to 4.4. The C/H ratio varied from 3.6 to 4.4 in eight analyzed samples. 2

6

The Reaction of Naphthalene with Excess AsF . Naphthalene (0.20 g, 1.6 mmol) was dissolved in hexafluorobenzene (5 mL) in an evacuated Teflon reactor. AsF was admitted to the vessel at room temperature until a total pressure of 1 atm was attained. Copious amounts of afluffypurple solid precipitated. The analysis was the following: C, 59.78; H , 2.81; and C/H ratio, 1.77. The solid was amorphous to X-rays. In another experiment, a small amount of AsF was added slowly to a solution of naphthalene in CH C1 (mol ratio of AsF to C i H was ca. 1:6). Above the surface of the solution, where an excess of AsF was present, the purple solid just described was formed. In the solution, however, a much darker, nearly black solid precipitated. Over a period of 1-2 h following removal of the solvent, both products became gray. No X-ray pattern could be obtained from these solids. 5

5

5

2

2

5

0

8

5

Oxidation of Coronene with 0 A s F . 0 A s F (0.342 g, 1.55 mmol) was placed in a Teflon F E P reaction vessel. A disk of Teflon filter paper was inserted above the solid, and coronene (0.464 g, 1.55 mmol) was placed on the filter. Sufficient S0 C1F was condensed into the vessel to cover the coronene. At 195 K, the reaction pro ceeded slowly; it reached completion in 1 h. The vessel was allowed to warm to room temperature, and volatile products were removed under vacuum after 1 h. The product was a green, free-flowing powder. The analysis was the following: [(C24Hi )o.97AsF6] C , H . The IR spectrum of this solid (Figure 2) contains, in addition to bands similar to those in neutral coronene, characteristic absorptions at approx imately 700 and 400 c m due to hexafluoroarsenate(V). The magnetic susceptibility was found to follow the Curie-Weiss law down to 12 K; the effective Weiss constant (^B,ef) = 0.36 and magnetic moment θ = -1.8 °. 2

6

2

6

2

2

- 1

Oxidation of Coronene with Excess C F A s F . C F A s F was prepared in situ from 0 A s F (1.0 g, 4.5 mmol) in S0 C1F. The reaction vessel was held at 77 Κ in a dry nitrogen-filled glovebag while coronene (0.15 g, 0.50 mmol) was added. The mixture was warmed to 195 K, and the reaction was allowed to proceed for 90 min. The product, obtained after removal of volatile compounds at room-temperature, was a dark-green friable powder. The analysis was the following: [(CMHiô.siAsFe] C, H . The IR spectrum of this solid (Figure 2c) is similar to the more coronene-rich 6

2

6

6

6

6

6

6

2




172

1600

1200

WOO

1000

600

800

400

Figure 2. IR spectra of coronene and coronene salts, a: coronene; b: (C HuUAsF , η = 0.97; and c: (C H ) AsF , η = 0.51 M

6

M

12

n

6

material described earlier, but the absorptions due to AsF are relatively more intense. The magnetic susceptibility exhibits Curie-Weiss behavior down to 6 K; μ , eff = 0.83 and θ = - 5 . 9 ° . 6

Β

Oxidation of Coronene with AsF . Coronene reacted rapidly to give green and blue-green free-flowing powders upon exposure to gaseous AsF in a variety of solvents and at varying AsF partial pressures. Solvents used included sulfuryl chlo ride fluoride, hexafluorobenzene, dichloromethane, trichlorofluoromethane, and l,l,l-trichloro-2,2,2-trifluoroethane. Arsenic trifluoride was detected as a reaction product by IR spectroscopy. The color of the solid thus produced seems to be a qualitative measure of the extent of oxidation; the more highly oxidized materials were bluer than the coronene-rich solids. Product compositions were determined by gravimetry on the basis of the assumption that arsenic is present only as AsF " and that coronene is present as neutral molecules or electron-oxidized cations. Ob served mole ratios of coronene to hexafluoroarsenate varied widely (4.0-0.25); the smaller values were associated with the higher concentrations of AsF . DebyeScherrer photographs of the polycrystalline powders (see box) show that the products 5

5

5

6

5


10.

RIGHARDSON ET AL.

173


X —ray Powder Diffraction Data (d-spacings) for Coronene Salts: M J , AsF 6

η = 0.51

10.92w, 9.52m 8.63vw, 7.57ms, 6.96s, 6.53s, 6.10w, 5.64w, 5.28s, 4.93m, 4.70m, 4.51vs, 4.44m, 3.98s, 3.52m, 3.33vs, 3.19w, 3.07vw, 2.78m Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 25, 2017 | http://pubs.acs.org Publication Date: December 22, 1987 | doi: 10.1021/ba-1988-0217.ch010

5

η = 0.89

12.42mw, 10.89w, 8.54m, 7.09vs, 6.49m, 6.08m, 5.43s, 5.14m, 4.79w, 4.58vvs, 4.36m, 4.17vw, 4.03m, 3.71vw, 3.54mw, 3.45mw, 3.35vs, 3.21s, 3.01w, 2.87w, 2.15w, 1.65w η = 1.88

14.37w, 10.86ms, 7.54vs, 7.04m, 6.54vs, 5.31vs, 4.94vs, 4.67w, 4.35s, 4.19s, 4.02s, 3.72w, 3.61w, 3.50w, 3.32vvs, 3.16w, 2.33w, 1.66vw η = 4.00 10.78w, 9.49s, 7.58vs, 6.47vs, 5.97vw, 5.25s, 5.11mw, 4.91w, 4.72w, 4.35s, 3.95s, 3.51s, 3.43m, 3.31vs, 3.20w, 3.16w, 3.06m, 2.77w, 2.66vw, 2.33m, 2.05vw, 1.96vw, 1.90w, 1.65vw, 1.47w

NOTE: W denotes weak, s denote strong, m denotes medium, ν denotes

very.

differ significantly over the composition range and that neutral coronene, if present, is incorporated into the structures and is not coexisting as a separate phase.

Results and Discussion Chemical syntheses of radical-cation salts by electron oxidation of neutral aromatic precursors require powerful oxidizing agents and stabilizing anions with high ionization energies (I) (e.g., A s F " , R e F ~ , S b F ~ , and S b F " ) (5). The stable salt of an aromatic cation of sufficient oxidizing strength can be employed as a synthetic reagent in the electron oxidation of other aromatic compounds with lower ionization energies. Thus, hexafluorobenzenehexafluoroarsenate(V), C F A s F , provides a convenient one-electron oxidizing agent somewhat less energetic than the dioxygenyl salt from which it is most easily prepared (10) [I(0 ) = 281 kcal/mol, and I ( C F ) = 230 kcal/mol]: 6

6

6

6

2

n

6

2

0 AsF 2

6

6

6

+ C F -» 0 6

6

2

6

+ C F AsF 6

6

6


(1)

174


The reduction product is the relatively inert and volatile hexafluorobenzene molecule. Moreover, C F A s F decomposes at room temperature to volatile products (5): 6

6

6

2C F AsF -» C F 6

6

6

6

+ 1,4-C F 6

+ 2AsF

8

5

(2)

This occurrence means that an oxidation can be carried out with an excess of C F A s F . The remaining oxidant is then allowed to decompose in situ at room temperature, and the volatile side products are removed under vacuum. This technique has been applied in the quantitative preparation of octafluoronaphthalenehexafluoroarsenate [I(C F ) = 204 kcal/mol]: 6


6

6

6

10

C F AsF 6

6

6

+ C F -» C F 1 0

8

6

6

8

+ C F AsF 1 0

8

6

(3)

In the two fluoroaromatic cation salts, the cations appear to be well-separated from one another by the anions. In C F A s F , each ion is surrounded by eight nearest neighbors of opposite charge in a distorted CsCl-type lattice. In the case of C F A s F , although details of the structure are not yet known, the symmetry and unit-cell dimensions seem to preclude an arrangement involving coplanar stacks of cations. Octafluoronaphthalenehexafluoroarsenate is exceptionally stable (de composition occurs at 395 K), and in light of the report (6) of conductivity in ( C H ) P F , we sought to prepare the fluoro analogue of this "synthetic metal". However, despite repeated attempts using a variety of approaches, we did not obtain such a material. Metallic behavior in partially charged organic stacks derives from bonding interactions that occur as a consequence of overlapping of the highest occupied molecular orbitals (HOMOs) and singly occupied molecular orbitals (SOMOs) of the stacked ring systems. This overlapping requires that the planar aromatic species be closer together than their van der Waals thickness of about 3.3 A , as has been observed in ( C H ) P F . Such a close juxtaposition of octafluoronaphthalene molecules, however, would also bring the electron-rich F ligands close together. This interaction is probably sufficiently strongly repulsive to offset the weak bond ing interaction between the electron-oxidized and neutral aromatic rings. Attempts to prepare a bis(hexafluorobenzene) salt were also unsuccessful. A s F is able to electron oxidize: 6

1 0

1 0

1 0

8

8

2

2

8

6

6

6

6

6

+

5

3 A s F + 2e~ 2 A s F " + A s F 5

6

(4)

3

However, its oxidizing power is weaker than that of C F . In the case of benzene, A s F and C H react quantitatively in H F or S 0 C 1 F (10) to give the colorless crystalline solid ( C H ) A s F A s F ~ : 6

5

6

6

6

6

+

2

6

2C H

6

5

2

2

+

6

+ 2 A s F ^ (C H ) AsF AsF 5

6

5

2

2

6

+ 2HF


(5)

10.

RICHARDSON ET AL.

175


With 0 or C F , however, benzene reacts to give poly(p-phenylene) derivatives. Although polymerization is never observed in the A s F reaction, some of the diphenylarsonium salt is always formed in the 0 and C F reactions because of the presence in the reaction mixture of A s F formed in the decomposition of the oxidizing agents. This finding suggests that the first step toward polymerization is electron oxidation of C H [ i ( C H ) = 212 kcal/mol] to C H , and that A s F is not able to achieve this oxidation: 2

+

6

€

+

5

2

+

6

6

+

5

6


6

C H 6

6

+

+ C F

6

6

6

6

5

6

6

(or 0

+

2

+

)

C H 6

6

+ C F (or 0 )

+

6

6

(6)

2

Subsequent abstraction of H by the anion may also occur: +

C H 6

6

+ AsF " -» C H - + H F + AsF

+

6

6

5

(7)

5

Such interpretations are consistent with the conclusions of other investigators as to the cationic nature of intermediates in the preparation of poly(p-phenylene) (11-12). As the number of fused or linked rings in a series of poly nuclear aromatic molecules increases, the ionization energies of the neutral molecules decrease [i(biphenyl) = 183 kcal/mol, I(terphenyl) = 181 kcal/ mol, /(naphthalene) = 187 kcal/mol, i(anthracene) = 171 kcal/mol, J(naphthacene) = 161 kcal/mol]. The polymer that results when benzene is reacted with the powerful oxidizers 0 and C F is readily oxidized by A s F (13,14). (AsF , although not capable of initiating the polymerization of benzene, can polymerize the more easily oxidized phenylene oligomers, including biphenyl). Although the aromatic compounds undergo hydrogen elimination read ily upon oxidation [benzene (15) and naphthalene (16) can be polymerized eleetrochemieally; binaphthyl is formed in the thermal decomposition of bis(naphthalene)hexafluorophosphate], the analogous elimination of F i n the fluoroaromatic compounds is not energetically feasible. For large, planar, fused-ring systems, the tendency toward coplanar stacking gives rise to behavior similar to that observed for graphite inter calation compounds. Coronene is rapidly oxidized by 0 A s F , C F A s F , or A s F : 2

+

6

6

+

5

5

+

2

6

6

6

6

5

nC H 2 4

nC H u

+ 0 A s F ^ (C H

1 2 n

) AsF - + 0

+ C F AsF -^ (C H

1 2 n

) AsF - + C F

1 2

2

6

l2

2raC H 24

12

6

6

6

2 4

2 4

+ 3 A s F ^ 2(C H 5

2 4

+

6

+

1 2 n

6

(8)

2

6

) AsF - + AsF +

6

(9)

6

3

(10)

The extent of oxidation and thus the observed stoichiometry vary widely in the materials prepared by the oxidation of coronene. The X-ray diffraction patterns of these solids are characteristic of the particular stoichiometrics;


176


however, they have common features. The most striking common feature is the presence in each pattern of a strong reflection with a d spacing of about 3.3 A , which is the thickness of a coronene molecule. This condition suggests the possibility that these materials are layered and that the anions occupy positions within and not between the layers. H elimination cannot be ruled out in the syntheses of the coronene derivatives just described, or for any other aromatic system. Although the materials reported here appear to be homogeneous and nonpolymeric, con taining arsenic only as A s F ~ , the magnetic susceptibility data are not easily explained in terms of purely ionic formulations. The observed electrical conductivity may, therefore, be due either to stacking of the cations or to linking through H F elimination at the edges of the planar ring systems, or to a combination of the two. Thus, the structures adopted by these materials are strongly influenced by the extent of oxidation and the sizes and number of anionic species present.


+

6

Acknowledgments This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under Contract No. DEAC03-76SF00098.

References 1. Richardson, T. J.; Bartlett, N . J. Chem. Soc. Chem. Commun. 1974, 427-428. 2. Tanzella, F. L . Ph.D. Thesis, University of California at Berkeley, 1980. 3. Zuchner, K.; Richardson, T. J.; Glemser, O.; Bartlett, N . Angew. Chem. Int. Ed. Engl. 1980, 19, 944-945. 4. Richardson, T. J. Ph.D. Thesis, University of California at Berkeley, 1974. 5. Richardson, T. J.; Tanzella, F. L.; Bartlett, N . J. Am. Chem. Soc. 1986, 108, 4937-4943. 6. Fritz, H . P.; Gebauer, H.; Friedrich, P.; Schubert, U. Angew. Chem. Int. Ed. Engl. 1978, 17, 275-276. 7. Bartlett, N.; Biagioni, R. N.; McCarron, G.; McQuillan, B.; Tanzella, F.; In Molecular Metals; Hatfield, W. E.; Ed.; Plenum: New York, 1979; pp 293-299. 8. McKee, D . E.; Bartlett, N . Inorg. Chem. 1973, 12, 2738. 9. Tanzella, F. L.; Bartlett, Ν. Z. Naturforscher 1981, 36b, 1461-1464.

10. Ionization Potential and Appearance Potential Measurements 1971-1981; U . S.

Govt. Printing Office: Washington, DC, 1982; (NSRDS-NBS 71). 11. Hsing, C.-F.; Kovacic, P.; Khoury, I. A. J. Polymer Sci., Polymer Chem. Ed. 1983, 21, 457-466. 12. Milosevich, S.A.;Saichek, K.; Hinchey, L.; England, W. B.; Kovacic, P.J.Am. Chem. Soc. 1983, 105, 1088-1090. 13. Shacklette, L . W.; Eckhardt, H.; Chance, R. R.; Miller, G. G.; Ivory, D . M.; Baughman, R. H . J. Chem. Phys. 1980, 73, 4098-4102. 14. Eckhardt, H.; Miller, G. G.; Baughman, R. H . Synth. Met. 1984, 9, 441-450. 15. Rubinstein, I. J. Electrochem. Soc. 1983, 130, 1506-1509. 16. Satoh, M.; Uesugi, F.; Tabata, M.; Kaneto, K.; Yoshino, K. J. Chem. Soc., Chem. Commun. 1986, 550-551.

RECEIVED

for review September 29, 1986.

ACCEPTED

February 12, 1987.


Comparison of Polynuclear Aromatic Hydrocarbon ... - ACS Publications

Recommend Documents